When water freezes:

Question

When water freezes:

BlackTom AI · Accepted Answer

Freezing is an exothermic phase change: water releases heat to its surroundings as it turns from liquid to solid.

1. Relevant knowledge points and concepts
- Phase changes involve latent heat: for freezing, the latent heat of fusion is released.
- For water at 1 atm, latent heat of fusion Lf ≈ 334 J/g (or 3.34×10^5 J/kg).
- During freezing the temperature of the water remains at the freezing point until the phase change is complete; energy extracted from the system is the latent heat, not sensible heat.
- Sign convention: when the system (water) loses heat to the surroundings, Q for the system is negative (heat is "lost" from the system).

2. Step-by-step reasoning
- Liquid water molecules have higher average freedom of motion; forming an ordered ice lattice requires lowering molecular potential energy through stronger hydrogen bonding.
- When bonds form, energy is released; that released energy must flow out of the water into the surroundings for freezing to proceed.
- Therefore heat flows out of the water (heat is lost by the water) and into the environment.

3. Calculation example with intermediate steps
- Take 100 g of water freezing at 0°C. Heat released Q = m · Lf.
- m = 100 g; Lf = 334 J/g.
- Q = 100 g × 334 J/g = 33,400 J.
- Interpretation: the water releases 33,400 J to the surroundings during freezing.

4. Why the correct answer is correct
- "Heat is lost" correctly expresses that the system (water) releases latent heat during the liquid→solid transition; the numerical example confirms a positive quantity of energy leaves the water.

5. Why the other options are incorrect
- "Heat is absorbed": incorrect; absorption of heat occurs during melting (solid→liquid), not freezing.
- "Heat exchange is the same": vague/incorrect; freezing involves a net heat release, not no change.
- "Heat is desorbed": desorption refers to release from surfaces (adsorption/desorption) and is not the correct thermodynamic term here; the standard phrasing is that heat is released or lost.

6. Conclusion confirming the answer
- Freezing of water releases latent heat to the surroundings, so the correct answer is: heat is lost.

When water freezes:Single choice

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